Continuous Glucose Monitoring vs HbA1c: A Deep Dive into CGM Data and How to Use It

L to R: Libby Johnson, RDN, LD, CDCES, iabetes Care and Education Specialist, Park Nicollet Clinic, Thomas Martens, MD, Medical Director, International Diabetes Center, Consultant in Internal Medicine, Park Nicollet Clinic; Gregg Simonson, PhD, Director, Care Transformation and Training, International Diabetes Center, Adjunct Assistant Professor, University of Minnesota Medical School, all in Minneapolis, MN.

The landscape of type 2 diabetes management is rapidly evolving with continuous glucose monitoring (CGM) continuing to demonstrate its power for improving glycemic control. At the ACP Internal Medicine Meeting 2025 in New Orleans, Thomas Martens, MD, Gregg Simonson, PhD, and Libby Johnson, RDN, CDCES, presented compelling evidence and practical guidance on implementing CGM for patients with type 2 diabetes using insulin in primary care settings.

The Case for CGM Beyond HbA1c

Martens began the session with a critical assessment of hemoglobin HbA1c (HbA1c) as the traditional metric for diabetes management. While HbBA1c remains valuable, CGM offers a more comprehensive view of glycemic patterns, including presentation of retrospective data that allows assessment of glucose patterns over time. Martens presented results from recent meta-analyses comparing CGM versus blood glucose monitoring (BGM) in type 2 diabetes that showed a statistically significant 0.33% HbA1c difference, favoring CGM.

Perhaps more importantly, Martens emphasized the growing body of evidence linking time in range (TIR) with diabetes complications. Multiple cross-sectional and longitudinal studies have demonstrated associations between lower TIR and albuminuria, retinopathy, peripheral neuropathy, cardiac autonomic neuropathy, abnormal carotid intima-media thickness, and higher cardiovascular mortality.

CGM Technology: Current Options and Landscape

Martens offered a look at the "landscape" of CGM systems now available, distinguishing between:

Real-time CGM (rtCGM): Patient-owned systems that continuously measure and display glucose data (eg, Dexcom G6/G7, Freestyle Libre 3+)

Intermittently scanned CGM (isCGM): Patient-owned systems that measure continuously but display data only when scanned (eg, Freestyle Libre 2+)

Professional CGM (pCGM): Clinic-owned systems placed on patients for 7-14 days (eg, Dexcom G6 Pro, Freestyle Libre Pro)

He also reviewed the recent clearance in the US of over-the-counter CGM systems as a significant development. The Dexcom Stelo and Abbott Rio are appropriate for people with prediabetes and diabetes who are not on insulin. The OTC systems do not have alerts for hypoglycemia making them inappropriate for those individuals, Martens stressed. Along with the Abbot Lingo, all 3 of the systems promote their use for general health monitoring.

Interpreting CGM Data: A Systematic Approach

Martens then turned the presentation over to his colleague at the International Diabetes Center (IDC) Gregg Simonson, PhD, who walked through the IDC's "Determine Where to Act" framework that provides a structured approach to interpreting CGM data and adjusting treatment based on glycemic patterns.

DETERMINE if action is needed: Review time in ranges

• Is TIR (70-180 mg/dL) >70%? Is time below range (TBR) <70 mg/dL <4%?

WHERE is action needed: Review the ambulatory glucose profile (AGP) curve

• Identify patterns of hypoglycemia, hyperglycemia, or variability
• Verify patterns across multiple days
• Ask the patient what THEY see in the AGP report

ACT on the data: Use shared decision making with the patient to adjust medications and lifestyle

• Focus on patterns of hypoglycemia first
• Make one change at a time
• Remember that every 5% improvement in TIR is clinically beneficial

Simonson reminded the audience to talk to patients about the retrospective data. Review the curves with them. Ask them questions about activities or food choices when glycemia is high or about how they are using insulin when levels dip below <70 mg/dL. The visual information available with CGM is offers valuable teaching opportunities.

CGM-Guided Management for Insulin Users

Simonson reviewed in detail a second IDC resource for individuals with type 2 diabetes who require insulin. The Clinician CGM Guided Management (CCGM) framework categorizes patients into 4 categories based on TIR and TBR:

• TIR >70%, TBR <3%: Continue current regimen - this is the ideal
• TIR >70%, TBR ≥3%: Address hypoglycemia
• TIR ≤70%, TBR <3%: Address hyperglycemia
• TIR ≤70%, TBR ≥3%: Address hypoglycemia immediately and consider referral for additional support from a diabetes educator or even to an endocrinology colleague,

The framework provides specific medication adjustment recommendations for each category, considering the patient's current insulin regimen (basal only, basal-bolus, or premixed).

Before using the CCGM TIR/TBR tables, however, Simonson advised on steps to ensure an individual's current medication regimen is optimal.

Step 1: Assess the patient for comorbid disease, ie, established atherosclerotic cardiovascular disease, congestive heart failure, or chronic kidney disease? These are indications for adding either a GLP-1 mimetic or SGLT-2 inhibitor therapy.

Step 2. D/C the S/U. Is the patient on a sulfonylurea as well as insulin therapy? If so, to avoid hypoglycemia, consider stopping the sulfonylurea if basal insulin is the current regimen and stop the medication if the patient is on either basal-bolus or premixed insulin.

Step 3. Assess changes based on implementing Steps 1 and 2. If no change has occurred, move on to the CCGM tables to further fine-tune insulin use based on of the 4 categories of glycemic control. If the optimization changes were effective, wait 2 to 4 weeks before moving on to the CCGM framework, Simonson advised.

Case Studies: Putting Theory into Practice

The presentation included detailed case studies that applied the CCGM framework to real-world scenarios:

Case 1: William - A 55-year-old man with T2D for 13 years, on metformin, semaglutide, and basal insulin. His CGM data showed TIR of 47% with no hypoglycemia (category 3, above). Following the CCGM guidance, his basal insulin was increased by 10% (from 42 to 46 units) along with nutrition counseling. At 3-month follow-up, his TIR had improved dramatically to 93%.

Case 2: Bernadette - A 61-year-old woman with T2D for 12 years, on basal-bolus insulin, metformin, and dulaglutide. Her CGM showed TIR of 39% with minimal hypoglycemia (Category 3). Her regimen was adjusted by increasing total daily insulin by 10% and redistributing to a 50:50 ratio between basal and bolus insulin.

Implementation Challenges and Solutions

Martens then addressed several practical aspects of implementing CGM:

Coverage and reimbursement: As of April 2023, Medicare has covered CGM for patients treated with insulin or with documented problematic hypoglycemia

CGM data access: Cloud-based platforms like Dexcom Clarity and LibreView allow for remote monitoring, though EMR integration remains a challenge

Patient education: Technology-naïve patients may need extra support with device connection and data interpretation

Conclusion

The integration of CGM into primary care represents a paradigm shift in type 2 diabetes management. Moving beyond HbA1c to focus on TIR and detailed glucose patterns, Martens and Simonson stressed, will allow clinicians to make more more informed and nuanced treatment decisions that will ultimately improve outcomes.

As Martens emphasized, "The goal for the AGP is: Flat, Narrow, and In-Range." With structured approaches like the CCGM framework, primary care physicians now have practical tools to interpret and act on CGM data effectively.

Note: Libby Johnson, RDN, CDCES, led the session audience in a hands-on demonstration that included downloading a specific CGM app to a smartphone and then applying a CGM sensor and generating data, which they were then instructed on how to interpret.